Many proposals have been made to encapsulate core materials that require protection until time of use in a protective covering. Cf. Nack, Microencapsulation Techniques, Soap and Sanitary Chemicals, Vol. 21, pp 85- 98, Feb. 4, 1970. Among process for making somewhat globular capsules that have been described in the technical literature and patents are coacervation and spray drying.
The coacervation process generally involves three steps: (1) formation of three immiscible chemical phases, a liquid manufacturing vehicle phase, a core material phase that can be dispersed or emulsified in the vehicle phase, as droplets, and a coating material phase, (2) depositing the coating material phase around the droplets of the core material phase and (3) rigidizing the coating to form the self-sustaining particles. The typical resulting particle is a globule of core material surrounded by a wall of coating material. Size may vary from about 5 to 5,000 microns. The core material may be liberated by mechanically breaking the outer wall by external or internal force, by degradation of the outer wall by melting, decomposition or dissolving or by diffusion of the core material through the wall. Particles produced by this method have found successful application in a number of industries, e.g., coated duplicating papers and sustained release drugs, but have not been widely accepted for flavors and fragrances in the food and cosmetic industry because they are relatively expensive and not rapidly soluble in water, Cf. Bakan, Microencapsulation as Applied to Pharmaceutical Products, Eastern Regional IPT Section, Academy of Pharmaceutical Sciences, Philadelphia, Pa., Oct. 4, 1968.
In the spray drying process particles are produced by a three step Operation comprising (1) forming an emulsion of the liquid core material in a solution, usually aqueous, of the normally solid coating material and (2) breaking up the emulsion into droplets of desired size, e.g., in a spray nozzle, from a spinning disc, or apertured centrifugal atomizer, and (3) removing moisture in a drying environment to solidify the coating material in the droplets to form solid particles. The drying environment may be hot drying air, e.g., in a spray drying tower, a dehydrating liquid, e.g., propylene glycol; a bed of dehydrating powder, e.g., dry starch powder; or the like. The particles produced by this process, while they may be of various sizes and shapes and may be "hollow" or "solid", are characterized by cellular structure comprising many dispersed globules of the core material in a matrix of the coating material. "Solid" in this context means that a particle has more or less uniform structure throughout, as opposed to the "hollow" form of particle which has a shell surrounding a void, but it does not imply absence of pores or cells in the body thereof. Particles or capsules produced by this method have been used commercially in many applications, including foods where the core material is a flavoring oil and cosmetics where the core material is a fragrance oil. Cf. Balassa, Microencapsulation in the Food Industry, CRC Critical Review Journal in Food Technology, July 1971, pp 245-265; Barreto, Spray Dried Perfumes for Specialties, Soap and Chemical Specialties, December 1966; Maleeny, Spray Dried Perfumes, Soap and San Chem, Jan. 1958, pp 135 et seq.; Flinn and Nack, Advances in Microencapsulation Techniques, Batelle Technical Review, Vo. 16, No. 2, pp 2-8 (1967); Merory, Food Flavorings, Avi Pub. Co. (1960), pp 274-277.
One of the best of the known processes for producing microcapsules involves spraying into a drying atmosphere globules or droplets of an emulsion or solution containing, in a continuous aqueous phase, a hydrophilic colloid such as dextrin or gum Arabic as the coating material, with the addition if necessary of an emulsifier, and a volatile or non-volatile core material of organic liquid, hereafter sometimes referred to as oil or oils, in a dispersed phase. The products of this process are dry, somewhat porous powders consisting of roughly spherical, convoluted particles with the coating material in the solid state and with the organic liquid either dispersed as minute droplets throughout the particle, or dissolved in a solid matrix, or both, depending on the compatibility of the oil and coating material.
In the conventional spray drying process of producing capsules the surface of the sprayed globule of the emulsion dries to form a solid outer crust almost immediately on contact with the drying atmosphere and further evaporation of entrapped moisture normally causes the particle to shrink, forming craters and cracks in the crust.
Capsule manufacture by this process of spray drying has been accompanied by loss of considerable proportions of oil by evaporation during spray drying and the capsules produced have been characterized by relatively high extractable oil. The maximum practical proportion of oil to wall material that can be used in the emulsion is usually limited (1by factors inherent in the mixture, particularly the ability of the aqueous phase to hold oil as the dispersed phase, and (2) by the losses in processing. These practical considerations have limited the oil to a fraction of the highest proportion the particles theoretically are capable of containing and such oil as is originally encapsulated can gradually escape from the dry particle, perhaps by diffusing through the relatively porous, cracked and cratered wall material. A quick test of the potential loss during storage may be made by determining, in the manner hereinafter described, the percent "extractable oil". While the mechanism of the oil losses has not been fully established, both modes of oil loss, i.e., the loss during spray drying and during storage, may be due to the relatively poor barrier afforded by the walls of the particles produced by conventional spray drying procedure using conventional wall materials.